1. Field of the Invention
The present invention relates to air conditioning systems that utilize refrigerants and a compressor, and particularly to air conditioning systems capable of alleviating excessive increases in refrigerant discharge pressure within a heating circuit.
2. Description of the Related Art
A known air conditioning system is disclosed in Japanese Patent Application No. 7-19630 and includes a compressor 1, a cooling circuit 51, a heating circuit 52 and a controller 83, as shown in FIG. 5. The cooling circuit 51 includes a condenser 55, a first expansion valve 57 and a heat exchanger 59 provided on a passage connecting a discharge port D to a suction port S of the compressor 1. High pressure refrigerant discharged from the discharge port D of the compressor 1 is drawn through the above respective devices and back to the compressor 1.
The heating circuit 52 includes a bypass passage 52a extending from the discharge port D of the compressor 1 to the heat exchanger 59. A second expansion valve 63 provided within the bypass passage 52a between the discharge port D and the heat exchanger 59. The high pressure refrigerant discharged from the compressor 1 is not directed to the condenser 55, but rather is drawn by the compressor 1 through the second expansion valve 63 and the heat exchanger 59. Such a heating circuit 52 is generally known as a hot gas bypass heater.
The operation of the cooling circuit 51 and the heating circuit 52 is changeably selected by opening and closing selector valves 53a and 53b, which opening and closing operations are performed by the controller 83.
Because the refrigerant discharge pressure is higher when the heating circuit 52 is used than when the cooling circuit 51 is used, the air conditioning system must operate in a high pressure state when the heating circuit 52 is utilized. An abnormally high-pressure state may be created if the output discharge capacity of the compressor 1 temporarily increases during the operation of the heating circuit 52. A refrigerant releasing passage 91 having a pressure relief valve 93 is provided in order to release excess pressure from the heating circuit 52, if an abnormally high pressure state is reached. The refrigerant releasing passage 91 is connected to the heating circuit 52 and the cooling circuit 51 and the pressure relief valve 93 can be opened to release the refrigerant from the heating circuit 52 into the cooling circuit 51 when the refrigerant discharge pressure abnormally increases during the operation of the heating circuit 52.
Because the cooling circuit 51 and the heating circuit 52 are alternatively selected by the selector valves 53a and 53b, the refrigerant is released into the cooling circuit 51 which is not used in operation of the heating circuit 52, thereby preventing the discharge pressure at the heating circuit 52 from increasing abnormally.
However, because the refrigerant in the heating circuit 52 is released into the cooling circuit 51 whenever the discharge pressure abnormally increases, the amount of the refrigerant in the heating circuit 52 is reduced and heating performance may be reduced. Moreover, because the high pressure refrigerant is wastefully released from the heating circuit, energy efficiency is reduced.
Another known variable displacement compressor is disclosed in Japanese Patent Application No. 10-47242. Although this compressor is not explicitly shown in the drawings, a connecting passage having a capacity control valve is provided between a discharge port and a driving chamber (also known as the crank case) in a housing such that refrigerant is released from the discharge port into the driving chamber when the capacity control valve is opened. The capacity control valve is opened to increase pressure in the driving chamber when the discharge pressure is high. On the other hand, the capacity control valve is closed to decrease the pressure in the driving chamber when the discharge pressure is low.
The output discharge capacity is decreased when the pressure in the driving chamber is increased, and the output discharge capacity is increased when the pressure in the driving chamber is decreased. The output discharge capacity is decreased to decrease the discharge pressure when the discharge pressure increases, and the output discharge capacity is increased to increase the discharge pressure when the discharge pressure decreases.
If the variable displacement compressor disclosed in Japanese Patent Application No. 10-47242 is employed in the air conditioning system having the hot gas bypass heater circuit disclosed in Japanese Patent Application No. 7-19630, abnormally high pressure can be solved without the insufficiency in the heating performance due to release of the refrigerant in the hot gas bypass heater circuit into the cooling circuit and without low energy efficiency due to wasteful release of the refrigerant at highly increased pressure by causing the compressor to work.
However, a problem can occur due to a combination of a characteristics of the variable displacement compressor and a characteristics of the hot gas bypass heater by only employing the variable displacement compressor disclosed in Japanese Patent Application No. 10-47242 in the air conditioning system having the hot gas bypass heater circuit disclosed in Japanese Patent Application No. 7-19630. As described above, because a step of opening the capacity control valve of the compressor to increase the pressure in the driving chamber, a step of decreasing the output discharge capacity and a step of decreasing the discharge pressure are necessary to alleviate the abnormal high pressure state of the discharge pressure in utilizing the technique disclosed in Japanese Patent Application No. 10-47242.
Therefore, if the discharge pressure increases drastically during these steps, the discharge pressure may abnormally be increased and the hot gas bypass heater circuit may be damaged by such abnormal increase in pressure because above-explained steps require considerable time to be completed.
The technique disclosed in Japanese Patent Application No. 10-47242 does not have a hot gas bypass heater circuit and has only the cooling circuit. In such a case, the pressure of the refrigerant flowing through the cooling circuit is inherently lower than the pressure of the refrigerant flowing through the hot gas bypass heater circuit, and the cooling circuit may not be damaged by abnormal increase in pressure.
In other words, the technique disclosed in Japanese Patent Application No. 10-47242 is sufficient when the hot gas bypass heater circuit is not employed. However, if the air conditioning system has the hot gas bypass heater circuit, because the pressure of the refrigerant during operation of the hot gas bypass heater circuit is inherently high, the output discharge capacity control technique of the known variable displacement compressor is insufficient to quickly alleviate the abnormal increase in pressure.